How do I fix these weird lines in my model
Those lines are a result of mesh distortion. It means that your polygons aren't "flat". Mesh distortion results in shading errors. 

Blender's Mesh Analysis functionality helps you checking with these kind of problems. In the screenshot below you see all the distorted polygons in your mesh. Grey is ok, then it goes from blue to green and finally to red (which means bad).



This is OK to a certain extend, but it depends on the purpose of your model. I would stay in the grey and blue-ish area to keep things safe.

The solution in your case would be working with a little more polygons, for example by adding loopcuts and then moving the vertices until the polygons change their color into blue or turquoise.